I. Field of the Invention
The present invention is directed to a solenoid actuated three-way valve of the type employed in pulse width modulated pressure control systems wherein the pressure at a control port of the valve is regulated by cyclically connecting the control port alternately to a high-pressure source and a low-pressure source to achieve a pressure at the control port proportional to the percentage of time during which the control port is connected to the high-pressure port.
II. Description of the Related Art
Valves of the foregoing type are being increasingly employed to control automotive transmission systems by regulating the engagement pressure of various clutches within the transmission to regulate the torque transmitted through the individual clutch. An on-board microprocessor receives various inputs representative of vehicle operating conditions, such as vehicle speed, engine RPM, throttle setting, etc. The processor is programmed to compute the optimum transmission ratio in accordance with the various inputs. In response to the inputs, the processor generates a pulse width modulated control signal at a constant frequency, typically in the order of 60 Hz, which controls energization of the solenoids of the various solenoid valves. During each cycle of the pulse width modulated control signal, the coil of a solenoid actuated valve is energized for a predetermined percentage of the cycle period and deenergized for the remainder of the cycle. Over a series of successive cycles, the pressure at the control port of the valve, assuming the pressure at the low-pressure port is zero, will be a percentage of the pressure at the high-pressure port equal to the percentage of time the high-pressure port is connected to the control port.
The requirements of a solenoid valve employed in such a system are basically that shifting of the valve armature between its alternate positions must closely and accurately track the rapidly repeated energization and deenergization of the solenoid coil. This requires the development of a maximum axial force applied to the solenoid armature upon energization of the coil, and minimum parasitic losses, such as friction, eddy currents, etc. within the assembly.
The present invention is directed to a solenoid valve which efficiently meets these last requirements, as well as the universal requirement of the automotive industry of low unit cost and simplified construction.